The present invention relates to a method and apparatus for inserting downhole tools into, and withdrawing them from, a pressurized wellbore and, in particular, to a lubricator that can be reciprocated into and out of a wellbore when it is mounted to a wellhead.
Inserting downhole tools through wellheads into pressurized wellbores has been practiced for decades, and is essential for certain phases of well drilling, well completion and well servicing. The process of inserting the downhole tools is generally accomplished using a tool commonly referred to as xe2x80x9clubricatorxe2x80x9d. A lubricator is one or more tubular members that form a sealed chamber around a downhole tool. The lubricator is attached to a pressure containment spool, such as a valve or blowout preventer at the top of the wellhead. At an upper end of the lubricator, sealing equipment such as a grease injector and/or a stuffing box seals the top of the lubricator, while permitting the downhole tool to be suspended by a downhole tool insertion string, a wireline for example, that extends through the sealing equipment. Thus, a sealed chamber is provided within the lubricator above a closure mechanism of the pressure containment spool. The sealed chamber houses the downhole tool and contains well pressure while the downhole tool is inserted into the wellbore. Pressure between the wellbore and the lubricator is equalized using a high-pressure hose connected between a bypass valve located below the pressure containment spool and a bleed port on or above the lubricator. The closure mechanism of the pressure containment spool is then opened, allowing access to the wellbore. The downhole tool is lowered into the wellbore by manipulating the downhole tool insertion string.
The downhole tool is extracted from the wellbore by drawing it up within the chamber in the lubricator, closing the pressure containment spool, venting the lubricator by opening the bleed port, and removing the lubricator and the downhole tool from the wellhead. This procedure works well for short downhole tools, and has been practiced for many years. However, some downhole tools are very long. For example, logging tools for monitoring the conditions within a production well are generally elongated tubular assemblies. Consequently, a lubricator for providing a sealed chamber to contain the downhole tool extends high above the wellhead. If a long lubricator is required, equipment mounted to the upper end of the lubricator is located even higher above the wellhead, making it difficult to access, awkward to operate, and placing considerable mechanical stress on the lubricator and the wellhead.
Efforts have been made to improve the method and apparatus for lubricating downhole tools into a wellbore. An example of such efforts is described in U.S. Pat. No. 4,681,168, entitled METHOD AND APPARATUS FOR RUNNING LONG TOOLS INTO AND OUT OF A PRESSURIZED ENCLOSURE, which issued to Kisling, III on Jul. 21, 1987. Kisling describes a method and apparatus for running long downhole tools into and out of pressurized enclosures including a tool stop assembled on an access pressure lock of the enclosure. The tool stop cooperates with a segmented tool string to allow sequential assembly, insertion, withdrawal and disassembly of the tool string. The tool string is made up of a number of tool segments interconnected by coupler/spacer members of a smaller diameter than the tool sections, and of a shorter length. A tool catcher acts upon the thinner sections of the coupler/spacer members to fixedly hold the tool string in place for subsequent assembly/disassembly without allowing any significant pressure change inside the enclosure. The lubricator used in Kisling""s method is therefore relatively shorter than the entire length of the assembled tool string.
Nevertheless, Kisling""s method and apparatus are limited to applications for lubricating tools that may be divided into tool segments. Tools used in well drilling, well completion and servicing are of several varieties, and not many of them can be lubricated through wellheads into wellbores using the method and apparatus described by Kisling. For example, a deep drilling motor, which is about ten feet long and is driven using a coil tubing string, cannot be lubricated into the wellbore by using Kisling""s method and apparatus. In order to insert the deep drilling motor and the coil tubing string, a coil tubing injector must be installed on the top of the lubricator. Such a lubricator system with the injector mounted to the top is positioned high above the wellhead and is consequently difficult to operate.
There therefore exists a need for an improved lubricator that addresses these problems and is adaptable for use with various downhole tools used in well drilling, well completion and well servicing operations.
It is therefore an object of the invention to provide a lubricator through which a lubricator tube can be reciprocated, in order to lower equipment mounted to the top of a lubricator tube to promote safe and convenient operation of such equipment and reduce stress on the lubricator and the wellhead.
In accordance with one aspect of the invention, a reciprocating lubricator provides a sealed chamber for accommodating a downhole tool to be inserted into a wellbore closed by a pressure containment spool. The reciprocating lubricator comprises a lubricator tube, a lubricator base and a lubricator head. The lubricator base is adapted to be mounted in a fluid-tight seal to a top of the pressure containment spool, and includes an axial passage for sealingly receiving the lubricator tube. The lubricator head has an axial passage and is sealingly connected to a top end of the lubricator tube, in order to provide a base for supporting other equipment to be mounted to the lubricator head. At least one lubricator injector is used to move the lubricator tube and the lubricator head from a first position in which the lubricator tube is positioned above a wellbore closure mechanism of the pressure containment spool, to a second position in which the lubricator tube extends down through the pressure containment spool in order to lower the lubricator head.
The reciprocating lubricator preferably comprises a lock mechanism for locking the lubricator tube and the lubricator head in the second position. The lubricator head preferably includes an adapter affixed to the top thereof for sealingly connecting the other equipment to the top of the lubricator head. The axial passage extends through the lubricator head and the adapter to permit a tool insertion string to pass therethrough for suspending the downhole tool accommodated in the chamber. The other equipment provides a seal around the tool insertion string while the downhole tool is lowered into the wellbore. The seal may be provided, for example, by a blowout preventer, a grease injection tube or a stuffing box, depending on the type of downhole tools and the type of tool insertion string required for a particular job.
In accordance with another aspect of the present invention, a method is provided for inserting a downhole tool into a live wellbore closed by a pressure containment spool. The method comprises mounting a reciprocating lubricator with the downhole tool accommodated in a sealed chamber of the lubricator to the top of the pressure containment spool. After the mounting of the lubricator with the downhole tool accommodated in the sealed chamber, the wellbore closure mechanism of the pressure containment spool is opened to allow access to the wellbore so that the sealed chamber can communicate with the wellbore. The lubricator tube with the downhole tool accommodated in the sealed chamber is then inserted downwards into the pressure containment spool, in order to lower the lubricator head. Thus, any equipment mounted on the top of the lubricator head is lowered together with the lubricator head.
After the lubricator head is lowered, the downhole tool insertion string is manipulated to lower the downhole tool to a required position in the wellbore.
The reciprocating lubricator can be used to insert various downhole tools, or strings of downhole tools, into wellbores. The reciprocating lubricator is particularly advantageous for coil tubing applications, because the reciprocating lubricator lowers the position of the coil tubing injector, which improves safety and relieves stress on the wellhead.
Other advantages and features of the present invention will be better understood with reference to preferred embodiments of the present invention described hereinafter.